The present invention relates generally to a method and apparatus for evaluating fluids and, more specifically, to a method and apparatus employing a conductivity sensor comprising at least two electrodes for the analysis of fluids subject to degradation and/or contamination and a spectrometer to detect the presence of wear metals and elemental constituents in the fluid.
Fluids, such as oils, lubricants, and other similar fluids are used in ways that cause their degradation. For example, it is common to lubricate and cool the components of operating equipment by wetting those components with an oil or lubricant. As the oil or lubricant performs its functions, it experiences environmental stresses which cause its base stock to degrade. As another example, oils used as transmission fluids and in hydraulic systems are subjected to stresses such as pressure, frequent movements, and heat. Stresses such as these also degrade the fluid.
Since it is undesirable to use a fluid beyond its useful life, an equipment operator will monitor a fluid and establish scheduled fluid changes for the equipment. The length of operating time between scheduled changes is chosen conservatively so that a fluid, which is beyond its useful life, does not remain in the equipment and damage the equipment. Unfortunately, this conservative approach results in fluids which still have useful lives being discarded. This approach also will not provide the operator with an indication of whether the equipment is operating properly.
Fluid condition monitoring is a technique that involves the analysis of a fluid in field use for the purpose of assessing either its level of degradation or its residual capacity to perform some important tribological function. The condition of the used fluid is determined by detecting changes in certain chemical or physical properties of the fluid caused by degradation of the fluid base stock or depletion of the additive package. For properly running equipment, the physical properties or chemical compositions of the fluid change at a certain rate, generally quite minimally, until the additive package is depleted. Changes in equipment parameters or conditions which can affect the fluid itself may or may not be coincident with changes in the level of wear debris in the fluid. For example, a fluid ages or degrades in normal operations, sometimes to an unacceptable level without any abnormal operation, of the equipment. Thus, monitoring can be used to assist in the determination of the proper interval for fluid changes.
When abnormal operating conditions occur, e.g., an increase in aeration rate of the fluid (caused by excessive seal leakage, deteriorated "O" ring, cracked diffuser cases, etc.) and/or an increase in fluid temperature, the rate of fluid degradation increases and the physical properties and chemical composition of the fluid change commensurately. In these cases, fluid monitoring can identify abnormally operating equipment which cannot be identified by the more frequently encountered monitoring of wear debris. In these situations, detections of fluid condition supplement wear metal analysis in detection of atypical mechanical system operation.
U.S. Pat. No. 4,029,554 to Ellison teaches an oil analysis method and an apparatus, known as Complete Oil Breakdown Rate Analysis (COBRA), for detecting when an oil is approaching the end of its useful life. In this method, a sample of oil is placed between two electrodes and an electric current is applied to the oil. The current causes an oxidation reaction to take place at one of the electrodes to form an oxide on one of the electrodes. This oxidation reaction produces a corresponding voltage output across a resistor and the output is measured. The greater the concentration of oxidation by-products present in the oil, the less resistance between the electrodes and the greater the current produced.
U.S. Pat. Nos. 4,744,870 and 4,764,258 to Kauffman, which are both assigned to the assignee of the present invention, disclose methods for determining the remaining useful life of fluids. These methods are fast, accurate, easy to operate and can be performed with inexpensive equipment. In these methods, fluid samples are mixed with a solvent and an electrolyte in either an organic base or a solid substrate, depending on the type of fluid which is being tested. A sample of fluid is removed from the system, placed in an electrolytic cell and subjected to a cyclic voltammetric analysis. The current generated during the cyclic voltammetric analysis is measured and recorded. The remaining useful life of the fluid is then determined from the oxidation or reduction wave height.
U.S. Pat. No. 5,071,527, also to Kauffman and assigned to assignee of the present invention, provides a more complete fluid analysis, including on-line analysis, which measures antioxidant depletion, oxidation initiator buildup, product buildup, and liquid contamination. The method uses a triangular wave form which is applied to a microelectrode. The resulting current is monitored at a second microelectrode. In high resistance fluids, such as oils, fuels, etc., a third electrode is used as a reference voltage for the applied voltage wave form. This technique uses a voltage range of about .+-.120V to cause electro-oxidation of antioxidants and electro-reduction of hydroperoxides and other thermal-oxidation products. The current flow produced by the electro-chemical reactions is then used to monitor the remaining useful life of the monitored fluid.
While the above techniques provide useful fluid analyses, they are stand alone analytical tools that provide a single data point that could only relate to changes in the condition of the lubricant. Thus, a need exists for a fluid analysis technique which requires less circuitry, can be conducted off-line, requires less time, and can be used in conjunction with other supplementary oil analysis techniques. Further, the above methods do not provide a method for analyzing the concentration of wear metals and elemental constituents in the fluid. Thus, a need also exists for a method which can simultaneously determine the content of wear metals and elemental constituents in the fluid and the condition of the lubricant.